The main wing comprises a wing leading edge, a wing trailing edge opposite to the wing leading edge, a wing root end which is configured to be mounted to a fuselage of a respective aircraft, and a wing tip end opposite to the wing root end. The wing tip device has an attachment end, a front blade and a rear blade. The attachment end is attached to the wing tip end of the main wing. The front blade has a front blade leading edge and a front blade trailing edge. The rear blade has a rear blade leading edge and a rear blade trailing edge. The front blade and the rear blade extend away from the attachment end in a diverging manner, i.e. they do not extend entirely in a common plane, although they might have sections extending in a common plane. The front blade leading edge extends in front of the rear blade leading edge, and the front blade trailing edge extends in front of the rear blade trailing edge, when viewed in a chord direction.
At the end of the wing tip device opposite to the attachment end, the front blade has a front blade tip, and the rear blade has a rear blade tip. On the front blade tip and on the rear blade tip, tip caps are attached for rounding and protecting the front and rear blade tips. At the front blade tip, the front blade extends under a different dihedral angle than the rear blade at the rear blade tip so that the front blade tip extends on a different level than the rear blade tip, when viewed in a wing thickness direction or along a yaw axis of an associated aircraft. The dihedral angle in general is measured with reference to a pitch axis of an associated aircraft in a plane spanned by the pitch axis and the yaw axis, or with reference to a horizontal plane when the aircraft is in a regular position on the ground. The dihedral angles at the front and rear blade tips, as well as all other angles defined at the front and rear blade tips under the present disclosure, are measured at the front blade tip and at the rear blade tip, respectively, right before the beginning of the tip caps, i.e. at the connection line to the tip caps, in particular as a tangent to the connection line.
The front blade leading edge and the front blade trailing edge both have a tangent-continuous developing without inflexion points or kinks. A tangent-continuous developing within the meaning of the present disclosure includes any continuously curved sections, as well as any straight sections which are connected to the continuously curved sections in such a manner that the developing of the tangent of the curved sections continuously transits into the straight sections, so that at the transition from the curved sections to the straight sections the tangent of the curved sections coincides with the straight sections. In other words, a tangent continuous developing excludes any kinks or discontinuities at the curved sections, straight sections, as well as at the transitions between curved and straight sections. Further, the rear blade leading edge and the rear blade trailing edge both have a tangent-continuous developing without inflexion points or kinks.
Wing tip devices in general are provided in the art in order to reduce the induced drag of a wing. Induced drag, also called lift dependent drag, occurs when lift is produced. Lift is produced by the wing through accelerating air mass downwards downstream of the wing with a certain momentum change equivalent to the lift produced. The continuous production of momentum consumes energy equivalent to the lift dependent drag. Minimizing induced drag requires minimizing the downward air deflection per span unit. Hence efficiency of the respective wing increases with higher span. In case of span limitations non-planar wings with wingtip devices help further minimizing the lift dependent drag. Due to wing flexibility upward pointing devices reduce the effective span more than downward pointing elements. Combining both elements while still respecting the need for a constant downwash distribution far downstream of the wing through the right aerodynamic loading helps to maximize the benefit. A constant downwash distribution represents a general optimization goal in order to minimize the induced drag a wing.
According to the prior art, various wings for an aircraft are known including wing tip devices as described above, or similar ones. For example, U.S. 2002/0162917 A1 discloses a wing comprising a wing tip device formed as a single blade which is curved upwards in a continuous manner. Similarly, U.S. 2010/0019094 A1 describes a wing tip device including a single blade which is curved upwards in a continuous manner leading into a straight tip section.
Also, double-blade wing tip devices are known in the art. For example, in the 1970's Richard T. Whitcomb developed a wing tip device which was later realized on the DC-10 and MD-11 aircraft. The wing tip device includes a small front blade which points downwards and a bigger rear blade which points upwards from the wing. Both blades are attached to the wing in a canted, discontinued manner which leads to an undesired complex flow in the junction region, in particular on the suction side of the wing, and consequently, undesired drag.
Similarly, in G. Loebert, MBB-UFE 1344, “Der Flügel mit Gabelspitzen als Mittel zur Erhöhung der Wirtschaftlichkeit von Transportflugzeugen”, 1977, a double-blade wing tip device is described comprising a front blade which extends upwards and a rear blade which extends downwards out of the wing plane. Yet again, both blades extend away from the wing in a cantered and discontinued manner, so that the specific flow challenges in the junction region which lead to undesired drag also apply here.
As a more recent example of a wing comprising a double-blade wing tip device, U.S. 2013/0256460 A1 discloses two fully overlapping blades, one pointing upwards and one pointing downwards out of the wing plane. However, also in this case the blades are connected to the main wing by hard, discontinued junctions, thereby challenging the flow in these junction areas. Further, both blades extend from a common root, so that they overlap over their entire span, which also causes undesired flow conditions.
From U.S. 2012/0312928 A1 a wing tip device is known having two blades, wherein an upper blade extends in a continuous manner upwards from the tip of the main wing. However, the lower blade, which is mounted to the upper blade in a fully overlapping manner, again extends away from the upper blade under a certain kink, i.e. in a discontinued manner, so that the flow in the kink area is undesirably challenged and drag is caused. U.S. 2013/0092797 A1 describes a similar wing tip device comprising an upper blade, which is formed continuously on the tip of the main wing and extends upwards in a straight way, and a smaller lower blade which is connected to the junction of the upper blade in a discontinued manner.
The “Morphing Airplane” developed by NASA comprises a wing including a wing tip device having two blades pointing upwards under a similar dihedral angle, wherein the blades are formed with the wing in a continuous manner, wherein a front blade exists in front of a rear blade such far that, however, they do not overlap at all and the wing tip has an undesirable large extent in a chord direction, and, thus, an undesirable high weight at the wing tip.
Finally, U.S. 2009/0084904 A1 describes a wing tip device having a front blade and a rear blade which are attached to the main wing in a subsequent manner, i.e. by a certain staggering. Yet, the staggering involves a gap between the front blade and the rear blade so that front and rear blade do not overlap at all. Further, again the front blade and the rear blade are attached to the main wing in a discontinued manner including kinks.